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Estimating relative humidity from outdoor air temperature and humidity, and indoor air temperature is a means to assess thermal comfort conditions when indoor relative humidity is not available. Buildings in tropical countries are usually well ventilated in the summer and show a large infiltration area. Considering that sources of moisture are minimal in such cases, indoor absolute humidity can be assumed to be the same as the outdoor value. However, if the indoor relative humidity is not available, it can be estimated from measured outdoor air temperature and humidity by converting these to absolute humidity, which would be assumed to be the same in- and outside. Provided that indoor air temperature is known, it is then possible to calculate relative humidity. In this paper, relative humidity estimates were compared with measured relative humidity values in four different low-cost houses in Curitiba-Brazil (latitude = 25o, altitude= 914m). This comparison comprises: a) a statistical analysis and b) the evaluation of comfort levels on the Building Bioclimatic Chart. In the statistical analysis, correlation coefficients were analyzed and in the evaluation of comfort, measured and estimated values were plotted in the chart and the percentage of hours within each zone was compared. The four different construction systems were: a wood board house, plastered on both sides; a reinforced concrete- panel house; a reinforced polystyrene plastered board houses; and, a house made of pre-molded pillars spaced with concrete prefabricated plates. These four houses were occupied during the monitoring period. Measurements (with HOBO data-loggers) were carried out in 15 days with a sampling time of 15 minutes.